The Moon: South Pole-Aitken Basin
Found on the far side of the Moon, the diameter of this impact crater is equivalent to the distance from London to Athens. The massive Aitken basin measures 2,500 kilometres (1,600 miles) across and is the largest, deepest and oldest basin on the Moon. In fact, it’s as deep as six kilometres (3.7 miles) in some places. For comparison, some of the largest impact craters on Earth are only several hundred metres deep.
The Aitken basin is thought to have formed about 4.3 billion years ago, just a few hundred million years after the formation of the Moon itself. Its origin, however, remains somewhat of a mystery. If it formed through a high-velocity impact then scientists would expect to find material from deep within the Moon’s mantle at the bottom of the basin, but this doesn’t seem to be the case.
Instead, it’s thought that a low-velocity projectile hundreds of metres across impacted the Moon at an angle below 30 degrees, enough to create the giant crater but not fast or steep enough to dig deep into the lunar surface.
However, the whole of this impact crater isn’t visible. For the largest crater that we can see all of, we need to head to the Red Planet…
Mars: Hellas Planitia
The largest visible impact crater in the Solar System is Hellas Planitia on Mars, a giant depression with a floor over seven kilometres (4.3 miles) below the Martian surface. Such is its breadth and depth that you could fit every Western European country inside it.
Material from the impact that formed Hellas Planitia stretches for up to a further two kilometres (1.2 miles) from the walls of the crater. It is thought to have formed about 3.8 to 4.1 billion years ago when Mars was hit by a number of objects during the Late Heavy Bombardment period in the Solar System.
Within the crater there are a number of interesting features that might make it an interesting place to visit on a future exploration mission. It contains gullies that would allow for the presence of liquid water if the temperature rose high enough, owing to their distance below the surface.
Radar images from the Mars Reconnaissance Orbiter also suggest that glaciers reside beneath layers of rock and dirt in three further craters inside Hellas Planitia.